Researchers at Cornell University in Ithaca, N.Y., and University of Massachusetts Medical School in Worcester have used laser tweezers to reveal the uncoiling of DNA in a nucleosome. The work, which they reported in the Feb. 19 issue of Proceedings of the National Academy of Sciences, suggests that DNA is released in three stages for reading by other molecules.

By applying force to a strand of DNA that was affixed to a coverslip and to a streptavidin-coated sphere that was controlled with the tweezers, the team found that low force released 76 base pairs, higher forces released 80 more, and still higher released the rest. If all but the last 10 pairs were released, the DNA could reassemble itself.